Peptide nucleic acid (PNA), first reported by Nielsen et al. in 1991, is a synthetic DNA/RNA mimic in which the ribose-phosphodiester backbone is typically replaced by an N-(2-aminoethyl)glycinyl (aeg) amide linkage and the nucleobases are linked to the α-nitrogen of aeg backbone through a methylene carbonyl moiety at approximately the same distance as in natural DNA/RNA (see FIG. 1A; or Nielsen, P. E., et al., Science (1991) 254, 1497). This molecular design allows PNA to recognize and hybridize complementary DNA/RNA through Watson-Crick base pairing with high affinity and specificity. Because of its non-peptide non-nucleic acid nature, PNA is not degraded by either nucleases or peptidases and therefore is very stable in vivo. Since PNA is made up of achiral, amino acid-like monomers, it can be easily prepared with well-established peptide synthesis protocols without problems of enantiomeric impurity. Because of these remarkable properties, PNA has the potential to become a leading agent for antigene and antisense applications.
However, like many large oligomeric compounds of similar nature, PNA has poor cell membrane permeability which has severely limited its use in biomedical research (Koppelhus, U., & Nielsen, P. E., Advanced Drug Delivery Reviews (2003) 55, 267). Considerable efforts have been devoted to improving PNA cellular uptake during the past decade. Using cell-permeable peptides as PNA-delivery vectors has been a popular approach. When attached to PNA, these peptides have been found effective to deliver the PNA cargo into living cells in various in vitro studies. Alternatively, a more appealing strategy is to design new PNA molecules with built-in cell permeability through incorporating the membrane translocation features of cell-permeable peptides onto the PNA backbone. Many studies have been published describing backbone-modified PNA analogues for this and other purposes. Among these, introducing an amino-acid side chain bearing a positive charge, such as that of lysine, ornithine or arginine at the α- or γ-position of the aeg backbone (FIG. 1B and FIG. 1C), results in improvement in water solubility and cellular uptake. Notably, PNA with either an L-Arg or D-Arg side chain at the aeg α-carbon, i.e., D-Argα-PNA and L-Argα-PNA, have been reported to have good cell permeability (Zhou, P., et al., J. Am. Chem. Soc. (2003) 125, 6878; Dragulescu-Andrasi, A., et al., Chem. Comm. (2005) 244). So far, almost all the modifications have been focused on the aeg backbone carbons which inevitably generate a chiral centre, and the two stereoisomers often exhibit rather different binding behaviours towards complementary DNA or RNA, possibly as a result of differential interstrand or intrastrand steric interactions caused by the two configurations (Sforza, S., et al., Chirality (2002) 14, 591). It is therefore of utmost importance to prevent epimerization during monomer synthesis and oligomer assembling to ensure the optical purity of the PNA analogues.
Accordingly, it is an object of the present invention to provide a PNA molecule with properties that overcome at least some of these disadvantages.